Optimization for adding and deleting in SymmetricGridWidget
Optimization for adding and deleting in SymmetricGridWidget

- avoids costly relayout in certain cases

pllim/ginga,naojsoft/ginga,pllim/ginga,pllim/ginga,ejeschke/ginga,ejeschke/ginga,naojsoft/ginga,ejeschke/ginga,naojsoft/ginga